The present invention relates to a composition capable of giving a high-performance plastic magnet having excellent magnetic properties as well as thermal properties resistant against air oxidation.
The permanent magnets as a major current include so-called sintered magnets prepared by the powder metallurgical techniques and cast magnets by casting a molten alloy into a mold. One of the serious problems in these magnets is that these magnet materials are not suitable for working into a very complicate form so that permanent magnets prepared by precision working are unavoidably very expensive. The distribution of magnetism in the permanent magnet of these types cannot be so uniform as desired. When a magnet with radial anisotropy or multipolar anisotropy is desired and prepared by these techniques, the magnet sometimes fractured so that yield of acceptable products usually cannot be high.
So-called plastic magnets have been developed to overcome these disadvantages and problems in the sintered and cast magnets. In the early stage of the development of plastic magnets, the powdery magnetic materials used to be bonded with a plastic polymer were mainly ferrite-based ones in view of the inexpensiveness of these magnetic materials. In compliance with the recent demand for more powerful and small-size or light-weight plastic magnets, the ferrite-based magnetic powders are under continuous replacement with metallic or alloy-type magnetic materials of which the rare earth-cobalt type magnetic materials are the most promising by virtue of their outstandingly high magnetic performance.
Although the magnetic performance of the rare earth-cobalt type magnet powder is unquestionably superior to that of the ferrite-based magnetic materials, these metallic magnet powders have a difficult problem when used as the base material of plastic magnets. That is, since molding of plastic magnets is performed usually at a relatively high temperature of 200.degree. to 250.degree. C. or higher so that the metallic magnet powder is rapidly oxidized in air at such a high temperature resulting in a great decrease of the magnetic properties. In some cases, there is even a danger of ignition of the magnet powder. The remedial means usually undertaken to overcome these problems are as follows.
(1) The procedure for the fabrication of plastic magnets is performed in an atmosphere of an inert or non-oxidizing gas. This method is considerably effective in preventing the oxidation of the magnet powder but complete prevention of air oxidation is rather difficult without decrease in the productivity and increase in the production cost.
(2) The magnet powder is subjected to a surface treatment in advance with certain coating agents such as titanium-containing or silane compounds. Such a surface coating is of course effective to prevent air oxidation of the magnet powder although complete prevention of air oxidation is also a very difficult matter. In particular, this method is almost ineffective when the processing temperature of the plastic magnet is 300.degree. C. or higher.
(3) The plastic polymer as the binder of the magnet powder is selected from those moldable at a relatively low temperature. This measure is of course effective in preventing air oxidation of the magnet powder so much to the extent of the decrease in the processing temperature. On the other hand, the upper limit of the temperature at which the plastic magnet is usable is naturally low and the magnetic properties of such a plastic magnet disadvantageously change or deteriorate relatively rapidly in the lapse of time.
(4) The loading amount of the magnet powder, i.e. the weight ratio of the magnet powder to the plastic polymer, in the plastic magnet is decreased. This measure, of course, cannot be undertaken when a high-performance plastic magnet is desired since the magnetic properties of a plastic magnet are directly affected by the decrease of the loading amount.